I have experimented with a lot of pattern paper, from store-bought pattern paper to newspaper to paper bags to tissue paper. The best? Shiny advertisements from the newspaper! Who knew? Here's why.
They are surprisingly durable
They are free (maybe this one should be first)
The fold up nicely but unfold flat too
They are full of pictures and writing (I just use a sharpie to draw the pattern over whatever's already on it).
Patter paper doesn't seem like a big deal, but if you've ever been annoyed half to death by trying to cut wispy store-bought patterns accurately, you'll know that the right paper makes all the difference. And since I am working on making 9 different sizes for my yoga pants pattern, you can well imagine that annoyance should be avoided :-)